Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices. These materials, however, may not be well suited for higher power and higher frequency applications because of their relatively small bandgaps (e.g., 1.12 eV for Si and 1.42 eV for GaAs at room temperature) and/or relatively small breakdown voltages.
In view of increased interest in high power and high frequency applications and devices, attention has turned to wide bandgap semiconductor materials such as the Group III nitrides, including gallium nitride (GaN, with a bandgap of 3.4 eV at room temperature). GaN also exhibits high breakdown fields of about 3 MV/cm, thus enabling such materials to withstand high power levels. In addition, GaN exhibits excellent electron-transport properties, which enables it to operate at high frequencies.
A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which, in certain cases, is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a thin layer of charge carriers, referred to a two-dimensional electron gas (2DEG), can form at the heterojunction of two semiconductor materials with different bandgap energies and electron affinities. The 2DEG can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2.
Homoepitaxial growth of Group III nitride based HEMTs on GaN substrates has typically focused on the use of semi-insulating GaN substrates. The lack of suitable GaN substrates, however, can make the growth of device quality heterostructures in this material system difficult.
Recent efforts have focused on the fabrication of Group III nitride type HEMTs using heteroepitaxial growth on substrates such as silicon carbide (SiC). The production of Group III nitride epitaxial layers (such as GaN layers) on silicon carbide substrates, however, can also be problematic. Relatively thin GaN epitaxial layers can exhibit electrical properties useful in various applications, including HEMTs. Thin GaN layers, however, can have unacceptably high dislocation densities, thereby rendering the structures unsuitable for many such applications.
Increasing the thickness of the GaN epitaxial layer can reduce dislocation density but to the detriment of other properties of the material. The increased thickness of the GaN epitaxial layer can, for example, adversely affect the electrical properties of the material, and in particular, can decrease the isolation and breakdown voltage of the material. In addition, GaN and SiC have different unstrained lattice constants (3.19 Å and 3.07 Å, respectively). The strain resulting from unmatched lattice constants can limit GaN epitaxial thickness and wafer diameter before the wafer cracks and/or bows, thereby rendering the GaN material unsuitable for downstream processing.